1. Field of the Invention
The present invention relates to a half-bridge inverter, and more particularly to a half-bridge inverter that accomplishes an inversion by switching an external-driven switch and a self-driven switch.
2. Description of Related Art
As electronic and computer devices are increasingly complicated, a power supply device becomes more important than ever. A power supply device can be divided into two main types: a linear type and a switching type. Since the linear type is less advantageous than the switching type, therefore, most power supply devices are the switching type.
The power supply for a backlight source of a thin film transistor (TFT) panel mainly uses a DC-AC inverter circuit to achieve the power conversion and drive a cold cathode fluorescent lamp (CCFL) to illuminate. With a different circuit topology, a conventional prior type of inverter circuit is generally divided into a half-bridge inverter circuit, a full-bridge inverter circuit, and a push-pull inverter circuit, which are inverter circuits for converting direct current (DC) into alternate circuit (AC).
U.S. Pat. No. 5,615,093 discloses a half-bridge inverter circuit. Referring to FIG. 1, a schematic circuit diagram of a half-bridge inverter circuit driving a load is illustrated. In FIG. 1, a transformer T1 divides a circuit into a primary circuit and a secondary circuit, and a feedback network 30 is connected between these two circuits. The primary circuit comprises a direct current power supply VDD, two electronic switches Q1, Q2, a half-bridge control chip 10 and a LCC resonant network 40, and the secondary circuit comprises a lamp load 20. With reference to FIG. 2 together with FIG. 1, schematic views of the output control signal of a half-bridge control chip and the voltage waveform of an alternate current (AC) according to a prior art are illustrated. The half-bridge control chip 10 uses two output ends LFET1, LFET2 to output control signals D1, D2, and the control signals D1, D2 respectively control the switching of two electronic switches Q1, Q2. By switching the two electronic switches Q1, Q2, the electric power of the direct current (DC) power supply is supplied to the primary winding of the transformer T1 through the LCC resonant network 40 to define an AC power supply ac. The AC power supply ac is provided for supplying power to the transformer T1, and the transformer T1 steps up and converts the AC power to the secondary winding for driving the lamp load 20.
In the foregoing description, the half-bridge inverter circuit disclosed in the U.S. Pat. No. 5,61.5,093 requires a half-bridge control chip 10 having at least two outputs to drive directly the electronic switches Q1, Q2 and control the alternating connection of the switches Q1, Q2. Further, the half-bridge control chip 10 also needs to control the first switch Q1 and the second switch Q2 to alternate for a short time by turning the switches on and off alternatingly. Therefore, it is necessary to have a little dead time for the control signals D1, D2 as shown in FIG. 2 to avoid a concurrent electric connection of the first switch Q1 and the second switch Q2, which will destroy both the first switch Q1 and the second switch Q2.
Further, the control signals D1, D2 need to change the signal cycle and control the switching of the first switch Q1 and the second switch Q2 to supply the electric power for the load according to the power requirement of the lamp load 20. The half-bridge control chip 10 needs to protect the circuit according to the conditions fed back from the load end, such as the condition of a too-low voltage output, a too-high voltage output, an open circuit of the lamp or overheat etc. The output of the control signal D1, D2 is terminated to switch off the first switch Q1 and the second switch Q2 to achieve a circuit protection function.
As described above, the conventional prior half-bridge inverter circuit comes with a half-bridge control chip 10 having two drive signals for driving the half-bridge circuit. Meanwhile, a protect circuit is needed to protect the secondary circuit of the transformer. Such requirement causes a high cost and an excessively large size of the product.